1. Field of the Invention
The present invention relates to a high-capacity anode active material which can improve the discharge capacity and life cycle characteristics of a secondary cell by controlling the shape of silicon particles through an electroless etching process using metal ions and by compounding a metal and carbon into shape controlled silicon, a method of preparing the same, and a secondary cell comprising the same.
2. Description of the Related Art
With the advent of the modern information society, power sources for portable electronics, such as mobile phones, notebook computers, personal digital assistants (PDAs) and the like, have been required to be miniaturized, highly energized and highly densified. Further, nowadays, as the oil crisis is returning, the interest in energy is increasing, and secondary cells used for hybrid electric vehicles (HEV) and the like are increasingly being used. Silicon, which is used as an anode material for lithium secondary cells, is a material replacing a carbon material.
Currently, a commonly-used graphite material has a theoretical electric capacity of 372 mAh/g, whereas silicon has a theoretical electric capacity of about 4200 mAh/g. However, in actuality, when silicon is fabricated into a silicon anode to make a cell, the cell only has a charging capacity of about 3260 mAh/g, a discharge capacity of about 1170 mAh/g and a coulombic efficiency of 35% (Electrochem. Solid State Lett., P.A306, Vol 7, 2004).
Further, when the cell is continuously charged and discharged over 5 cycles, its discharge capacity is rapidly decreased to about 300 mAh/g, which is about 10% of its initial discharge capacity. The reason for this is that, at the time of inserting lithium, a lithium-silicon alloy (Li—Si alloy, Li22Si5) is formed, thus causing a fourfold volume expansion. Owing to this volume expansion, the silicon structure breaks down, so that the electron pathway of an electrode is blocked, with the result that dead volume is formed in the electrode, thus causing the reduction in capacity of the silicon anode. Therefore, as the cell is continuously charged and discharged, its capacity is rapidly decreased. Such a life deterioration phenomenon significantly occurs in a bulk silicon film or particles of micrometers in size.
In order to solve the above problems, various methods have been proposed. For example, silicon was grown on the surface of an electric accumulator in the form of a wire, and was then used as an electrode. This method is advantageous in that electrons move easily compared to a conventional thin film, and the inner stress of silicon occurring at the time of charging and discharging a cell is decreased, thus exhibiting excellent cycle performance, but is disadvantageous in that it cannot be easily put to practical use because it is performed based on a thin film forming process (NATURE Nanotechnology, P. 31, vol 3, 2008).
Further, a method of controlling the shape of silicon nanoparticles has been proposed. In this method, silicon nanoparticles were formed into hollow silicon nanospheres, and thus the weakness of the silicon nanoparticles over cycles was overcome. However, this method is problematic in that the time taken to form the hollow silicon nanospheres is increased, and the process of forming the hollow silicon nanospheres is complicated (Adv. Mater., p. 4067, vol 19, 2007).
Further, Korean Unexamined Patent Application Publication No. 1999-0042566 discloses a method of forming porous silicon used for light-emitting materials by electrochemically etching a silicon wafer. In this method, a silicon wafer is immersed in an etchant, and then electric current is applied thereto to corrode the surface layer of the silicon wafer, thereby forming porous silicon.
Furthermore, a composite of silicon and silica was formed, and then only silica is removed from the composite to form many pores in bulk silicon particles, thereby overcoming the weakness of the bulk silicon particles over cycles. This method is different from a conventional method in the point that pores for alleviating volume expansion occurring in the silicon particles are formed by chemically etching just the silica. However, this method is also problematic in that the time taken to form the pores in the bulk silicon particles is long, the process of forming the pores in the bulk silicon particles is complicated, and a high-temperature heat treatment process must be performed (Angew. Chem. Int. Ed. p. 1, vol 47, 2008).
The present invention is greatly different from this method in the point that silicon particles are only partially etched for a short period of time to prepare a bundle type silicon nanorod structure.